Self-priming centrifugal pump



July 18, 1939, H. E. LA BouR 2,155,358

SELF-PRIMING CENTRIFUGAL PUMP 1 Filed D90. 29, 19:55 '5 SheebS-Slvleet 1 v July 18, 1939. H. E. LA BouR 2,166,358 Y SELF-FRIMING GENTRIFUGAL PUMP Patented July.18,1939

UNITED STATES Azitesxsz;

PATENT oFFlcE 2,166,358 SELF-PRIMING CENTRIFUGAI. PUMP Harry E. La' Bour, Elkhart, ina. Application December 29, 1936, Serial No. 118,032

6 Claims.

My-invention relates to self-priming centrifugal pumps of the type which is provided with a gas separator on the discharge side of the impeller casing, and wherein a body of priming liquid is retained in communication with the impeller and is circulated from the impeller chamberrto the separator and back again, being caused to carry gas out of the impeller casing by en- 'trainment and reentering the impeller chamber after gas separation has occurred, at such a point where the centrifugal effect of a full complement of liquid in the impeller is able to hinder or prevent reentry or if desired, produce outflow.

The chief object oi the present invention is to provide a simplified and inexpensive structure for carrying out the above stated functions in an eiiicient manner.

A, further object of the invention is to provide v a structure wherein the above'stated functions may be performed with large open passageways suitable for handling of solids in the liquid to be pumped, but with rapid and efficient evacuation of air or other gas from the pump body and suction line.

A further object of the invention is to 'provide a simple structure wherein a single large opening between the impeller casing and the separator permits simultaneous discharge of liquid with entrained gas from the impeller casing, and reentry of liquid for mixing with gas in the impeller casing with minimum interference between said streams. 5

A `further object ofthe invention is the provision of guiding means disposed in the separator and below the level of liquid therein for keeping A the issuing stream'of gas and liquid andthe reentering stream of liquid from material interference with-'each other, all withoutA opportunity for the lodgement of solids or stopping up any pasv-- `sageways. l

A further object of the invention is the provision oi' a. pumpA ofthe above class with eiicient means for guiding the circulation of the working ,medium during priming to enhance gas evacuation from '1 the impeller casing and improve separation of gas from liquid in the separator.

.Aiurther object of the invention resides in the provision of a new method of compensation and action in prior pumps, but by change of direc- .tion or diversion of the reentry stream;

will be free of recirculating such debris.

A further object of the invention is to reduce the cost of manufacture of pumps of this class.

A further object of the invention is the provision of a pump particularly suitable'for the construction industry, as well as for other uses.

In a self-priming pump of the 1character above stated, the effectiveness on priming and the degree of vacuum which can be drawn is dependent chiey on two factors. The rst is the effectiveness of separation of gas from the liquid in the separator so as to limit the amount of air carried back into the impeller chamber. Such air as is carried backexpands during the reduction of pressure in the impeller chamber and in addition to constituting a useless `circulation tends Ato reduce the entrainment of additional gas by liquid in the impeiler chamber. The pump of my present invention provides a novel separator and method oi separation.

The second feature is the matter of control of the density of the working medium during priming, that is, mixture of gas and liquid to develop the required difference in pressure between the central-part of the impeller and the periphery of the impeller. The present invention provides a scheme or method of compensa- -ity is kept high throughout the entire range.

Other objects and features of novelty will be aparent to those skilled in the art from the following specication anddrawings which are hereto attached, and I do not intend to disclaim them by failure specifically .to mention them.

Now in order to acquaint those skilled in the art with the manner of constructing and using a specific embodiment of my invention, I shall describe in conjunction with the attached drawings a specicform of pump in which my invention-appears, and shall describe the method of operation which it embodies and which is capavble of embodiment in other forms of pumps.

In the drawings: v 1

Figure 1 is a side elevational view of a pumping unit comprising a pump and driving motor mounted on a common' base;

Figure 2 is a vertical cross section taken on the line 2-2 of Figure 3;

Figure 3 is a section taken on the broken line 3 3 of Figure 2;

Figure 4 is a horizontal cross section taken on the line 4 4 of Figure 3;

Figure 5 is a fragmentary vertical section similar to that of Figure 3, showing a modified form .of pump employing a closed impeller; and

Figure 6 is a diagram provided for purposes of explanation.

It is to be observed that while` I have shown a pump of the volute type this is by way of illus` tration only, and it is to be understood that the invention may be embodied in a pump of the type wherein the runner sweeps the channel or a substantial portion thereof. Also, while the impeller herein shown is a three-blade impeller,

the number of blades is not to be construed as of the essence of the invention, more or less blades being permissible within the invention. The

l form of the blades may also be varied, and the impeller may be of the open or the closed form within the invention. Both forms are well known and need not be further described.

In the pumping unit shown in Figure 1, the pump unit I andthe direct connected motor 2 are coupled together through a bracket member 3 and are supported upon a suitable base plate 4. The motor base 5is clamped to one Aend of the base lplate 4, and a mounting bracket 6 of angular formation has a horizontal leg 1 disposed under and 'clamped to' a pad 8 formed under the intake trap 3 (see Figure 3). The vertical leg I0 of the bracket-61s clamped to the vertical face of the projection I2 formed on the base plate 4. Obviously, this specific form of support is capable of modiiication, its function being to steady the pump body I on the base 4, and to support the weight vof the pump body and its contained liquid and connected parts upon the base, while the bracket member holds the pump and the motor rigidly in coaxial relation. The pump may be supported on a separate bracket and pedestal construction of known form.

The motor 2 shown herein is a variable speed electric motor of the enclosed explosion proof type, such as is employed where ignitable vapors or the like may be encountered. The specific form of the motor is not of consequence. A gas engine or other driving means may equally' well be utilized.

In the specific construction shown, the bracket 3 has a flange I4 frequently termed a bell for clamping to the rim of the housing I5 of the electric or other driving motor. l

'I'he pump body I comprises a single casting in which three functionally separate chambers are formed. The rst is the trap 9. The second is the separator I6, and the third is the impeller casing which in the specinc form shown includes a volute. The intake trap 9 has a iianged neck I1 through which the inlet opening I8 is formed. The separator I6 likewise has a discharge neck I9 through which a discharge opening 20 is formed. These openings I8 and 20 are disposed on substantially the same horizontal level, and in the form shown are in the'same line. Through the impeller chamber I6a, the trap 9 and separator I6 form a U-shaped liquid trap for retaining a charge of priming liquid.

While the openings I8 and 20 are shown as facing .in opposite directions parallel with the impeller shaft, it is to be understood that this is merely the preferred disposition of these connections and disposal of the openings at either of the sides or top for either the intake chamber 9 or the discharge chamber I6 i's permissible within the invention.

The body I comprises a single casting wherein the intake chamber is defined by a vertical at front wall 22 (see Figures 3 and 4), vertical side walls 2 3 andV 24, and a common vertical partition 25 between the intake trap 9 and the separator I 6. The separator I6 is likewise defined by the vertical wall 26, which is parallel to the partition 25 and to the front wall 22 of the separator. The side walls 21 and 28 are direct continuations of the walls 23 and 24. In the vertical direction, the trap 9 has a top wall 29 which is substantially tangent to the outlet opening I8.

The bottom wall 38 of the trap 9 is shown as a semi-cylindrical wall tangent internally to the inlet opening 32 into the impeller casing 16a. Obviously, the form of the lower wall may be varied, but it should be so disposed that no lodgment is provided for debris, sand, gravel, or the like, which may be carried in with the liquid. That is to say, the bottom of the separator should be so formedthat the incoming liquid during pumping sweeps the bottom of the trap clear of solids, such as sand, gravel and debris.

It will be noted that the intake trap 9 and separator I6 are of similar cross section, that is, oblong rectangles, with the outermost corners rounded for the sake of appearance. o f cross section of the intake trap is not essential but it provides a suitable continuation of the common walls and gives symmetry to the construction.

The oblong parallelogram 'form of the cross section of the separator I6 is designed to produce a definite guiding action of the stream of gas and liquid discharged from the impeller casing Ilia in substantially the vertical plane of the impeller in a reverse loop. The top wall 34 of the separator is internally tangent to the discharge neck I9, and is generally cylindrical in contour, arched upwardly. This is not essential, since during priming the level of the water lies below the bottom of the dischargevopening 20. However, on

lstarting the priming action, water is sometimes thrown vigorously to the top of the chamber I6, and in that event the curvature of the top wall tends to guide the liquid back in a reverse loop with minimum disturbance or loss of liquid. The inlet and outlet necks I1 and I9 may slope upwardly and outwardly.I

The impeller casing I6 comprises an integral front Wall 35 which has the inlet opening 32 therethrough and the inside of the wall 35 is machined, as indicated at 36, to give a flat planar face of generally circular contour along which the front face of the impeller 3'I`sweeps. iinished surface 36 is relieved where it merges with the volute passageway 38, as indicated by the shoulder 39. The volute I6 extends from the cutoff point 40 in the form of a spiral passageway of increasing cross section to the point 42 where the volute terminates in a discharge opening 43.

The bottom wall 44 of the separator IB is disposed below the top Vof the volute on the two sides and in front thereof, as clearly shown in Figures 2, 3, and 4. The back part ofthe volute 45 which projects above the bottom wall 44 of the separator forms a baille for the liquid in the This 'I'his form separator I6. The side walls of the volute terminate in edges 46 and 41, and lie upon the opposite sides of the opening 4B in the bottom wall 44, which opening 49 lies immediately in advance of the cutoff point 40. The impeller 31 comprises a supporting plate or disc 49, and in the form shown in Figures 2, 3, and 4, three curved blades 50, 50, 50. The inner ends of the couplingv 54 to the driving shaft of the motor 2 or other driving means. The integral pump body 4i has a lateral circular opening or recess 51 on the back side of the impeller casing, that is, on the right as viewed in Figure 3, which opening 51 is surrounded by a neck 55 terminating in a clamping flange 5E. The opening 51 through this neck into the sideof the impeller casing is machined to a size suitable for the introduction of the impeller 31 and to provide for the location of a back plate having a side plate surface 58 matching the surface 36. The back plate 59 is recessed at its center to receive the mounting plate 49 of the impeller and is provided with a suitable hollow boss ill for housing the packing 62 and receiving the end of the follower 63, oo nstituting -a packing gland. The side plate 59 is cylindrical in contour to fit the finished surface 51 of the neck 55 of the pump body, and is provided with an integral flange 64' registering with the clamping ange 56, and adapted to be secured thereto by suitable cap screws or other fastening means. The extending arms 65, 65 of the bracket portion 3 provide openings B6 at each side through which access to the `packing gland may be had. These arms 65, 65 are integral with the mounting flange or bell I3. A suitableA gasket61 is clamped between the flanges 64, 56 to seal the joint between them.

In the modification illustrated in Figure 5, the front wall 35 is formed in the shape of a truncated cone to clear the front wall of the closed impeller 31'. The closed impeller has a circular inlet neck portion 68 which ts closely at 69 the central opening in the front plate 35'. The closed impeller may have straight or curved passageways leading from the eye 68 to the outer periphc'ry thereof, for discharge of the medium pumped into the volute 38, and thence into that part` of the volute which extends into the separator defined by the walls 45, 4S and 41, as in the con-f i struction shown in Figures 2, 3 and 4, and thence to the separator proper. A drain plug 68 permits draining of the water from the pump casing, including both the trap Stand the separator I6.

The impeller 31' is shown as beingsubstantially tangent to the bottom wall 44 of the separator. This is the preferred location, but it is to be understood 'that raising the periphery of the impeller slightly above or disposing it slightly below the wall 44, or raising or lowering the cutoff point 40, is permissible to meet requirements of design or to modify the rate of priming, or secure the desired action of compensation, without departure from the invention.

The operation of the pump shown in Figures 1 to 4, inclusive, -is as follows, assuming that a. suction line is connected to the intake neck I1 and a discharge line to the discharge neck I9.

If no liquid has been trapped in the pump. a suitable priming charge lmay be inserted through either one of the openings I8 or 20, or the pump may be lled by a lling plug optionally provided in either the intake trap or the separator, but not here shown. The level of trapped liquid will lie at a point below the bottoms of the openings Il and 20, and will be adequate if it lies at the level indicated by the line 4-4 in Figure 3. If now the rotation of the impeller is begun. the blades 50, 50 drive liquid out of the volute opening 43,and the communicating opening 48 until all of the water has been drawn from the intake trap. 9 and discharged into the separator. Thereupon air enters the eye of the impeller from the intake trap 9 and suction line, not shown. Assuming that the end of the suction line is submerged in liquid to be pumped, the continued operation of the impeller creates an entrainment of the air with liquid in the impeller casing, and

the discharge of liquid mixed with air out of' une` volute opening 43 into the separator I6, andthe simultaneous return of liquid from the bottom of the separator through the opening 48 into the perphery of the impeller to supply the impeller with liquid for such entrainment and discharge.

If no suction load is applied to the pump, as if, for example, the intake is open to the atmosphere, al1 the pressure difference that is required to be developed between the intake side and the discharge side of the impeller is that which is necessary to overcome the small static head of liquid above the impeller. For this slight difference, the density of .the working iiuid, i. e., mixture of gas and liquid, may be quite low, in some cases not much more than air with a small spray of water therein will be enough. As soon, however, as the suctionl line is closed, so that a partial vacuum is created in the center of the impeller by carrying out of some of the gas, the reentry of liquid from the separator into the impeller becomes more vigorous, thereby supplying a denser medium to develop the necessary difference in pressure between the intake and discharge of the impeller, It is known that the pressure developed at the blade tips is directlyproportional to the density of the medium operated upon, and the square of the velocity of the blade tips, all other factors of design remaining p the same. The mixture of gas and liquid which ture substantially in the plane of the impeller and in the plane of the separator, as will be ap- 'parent from Figure 4. I'he tendency for this discharged mixture is to pass above the cut oir. pointv 40, although due to the wide spacing of the 4blades 50, 50 in the form shown in Figure 2, there is a tendency, as the blades approach the cut oil!- portion 40, to pull some of the mixture towards the cutoff point `as each blade passes and discharge the same thereon. However, the main stream of discharge is maintained in a diagonally upward direction, asviewed in Figure 2 to the right, and upward where theV liquid by gravity separation and agglomeration of bubbles tends to free itself of the entrained gas, and is carried over towards the left, as viewed in Figure 2, and descends at the left side inside the wall 28, forming a reverse loop and moving in a counterclockwise direction. Where a greater number of blades is employed, the pulsations due to the blades sweeping toward the cutoilare more rapid, and less extensive in angular extent, with the result that a sharper and more localized region of reentry is dened in advance of the cutoff.

The densest liquid in the separator will be` found generally in the lower left hand corner of the separator, as viewed in Figure 2, back of the wall 45. This denser liquid, that is, the liquid having a minimum of entrained air, tends to drop down into the channels 12 and 13 on each side of the upwardly projecting part of the wall 45, and to be led in parallel streams on each side of the stream 0f mixture issuing through the opening 43. As the uppermost blade 50 reaches the region of the cutoif point 40, it presents on its back side a region of lower pressure into which liquid tends to be drawn by the partial vacuum and to be forced by the static head of the liquid in the separator. In order for these repriming streams of dense liquid to enter the periphery of the impeller they must change direction sharply, passing around the edges of the walls 46 and 41 and dropping down into the region of reduced pressure immediately below. In other words, this dense liquid is led along the side walls of the projecting part of the volute on each side of the plane of the issuing stream, and then the two streams pass toward each other under the issuing stream and enter into the impeller.

This is indicated diagrammatically in Figure 6. The impeller 31 is shown as rotating in a clockwise direction, as it is intended to do in Figure 2, as indicated by the arrows, and from the impeller issues the stream A of mixture which is guided upwardly and to the right until along the portion of the path V the mixture is compelled in move substantially vertically. Here the entrained air agglomerates and gravitates'upwardly. 'Ihence the stream of liquid turns back horizontally along the portion H and drops down vertically along the portion B towards the lower left hand corner of the separator as viewed in Figure 2. Here the liquid is compelled to divide into two paths C and D where it is guided in the channels 12 and 13 in planes parallel to the plane of the issuing stream, and on each side thereof. Immediately beyond the edges 46 and 41, as indicated by the lines E and F, the denser liquid flows around the edges 46 and 41 laterally and drops into the opening 48, which opening 48 is substantially tangent to the peripheral travel of the impeller blades 50, 50. Now it can be seen that to prevent reentry of the priming liquid the lines E and F need only be deflected in the direction of the issuing stream A..

'Ihe inclination of the edges 46, 41 may be varied to meet the particular requirements of the design, depending upon impeller speed, number of blades, rate of discharge and the like, so as to give more or less freedom for the returning heavy liquid to flow laterally around the edges 46 and 41 into the said part of the opening 48. The channels 12 and 13 are open at their tops, and hence no debris or solids can be caught therein. There is no reversal of ow in the channels 12 and 13, but they are open and tend to clear themselves by the continuous flow of liquid along the same.

It will be apparent that two channels and two streams are preferred as filling of the impeller is more rapid and compensation is more easily attained. But it is to be noted that a single channel for conducting the priming liquid around and under the issuing stream A may be utilized, if desired.

During priming as the vacuum load on the intake increases, the tendency for liquid tobe forced laterally and downwardly into the periphery of the impeller increases, but due to the nature of the reentry, that is, by a sharp change of direction under the issuing stream, compensation, that is. control of reentry in accordance with the requirement for density of the discharging medium, is very satisfactory. That is to say, the flow of fluid out of the impeller if it increases in volume or density tends to counteract the increased pull of vacuum as the vacuum is increased during priming with a very nice adjustment between the reentry of liquid to give the necessary density of the medium operated upon without drowning" the pump. So far as I am aware, this is a wholly new action in pumps of this class, and the method involved consists in introducing the reentry liquid through the single communicating opening by conducting the stream or streams through a sharp change in direction into a path which runs transversely to the direction of the issuing stream, whereby, upon any increase in density or volume of the discharge, the reentry streamer streams are diverted and the freedom of entry thereby curtailed.

It will bc observed that the path of the liquid during priming is that generally of a ligure eight, and that where the two streams cross at the neck of the figure eight, in the preferred form of the invention, the returning stream is divided and passes around the issuing stream and each branch comes back into the plane of the impeller and the piane of the issuing stream by a sharp change of direction of ilow in each reentry stream.

Obviously, one of these channels may be omitted, that is, reentry may occur on one side alone. 'I'his is readily accomplished by moving the wall 25 substantially in line with the edge of the wall 4S, or by other expedients, such as raising one edge of the volute, so that the issuing stream is directed slightly to one side or the other, and the reentry liquid then directed in from the other side only. The essential action is that of leading therentry liquid into the periphery of the impeller, by a sharp change of direction, so that the issuing stream may easily divert the rentry stream for compensation or for the nal stoppage of reentry when the pump receives its full complement of liquid.

During priming the mixture of gas and liquid is driven under the iniluence of the impeller blades, and being under the influence of centrifugal force, it tends to hug theA inner periphery of the impeller casing channel. This channel is sufficiently divergent adjacent the termination thereof that in effect it lifts redieuy from the impeller the stream of mixture to the extent that a region of reduced pressure is caused beneath the issuing stream driven out of the opening 43. The pressure prevailing in this region because of the outward divergent travel of the stream of mixture is low enough to cause liquid in the lower part of the separator to ow into it and engage the impeller blades in this region, which is preferably slightly in advance of the cutoif point 40. Liquid from the separator being thus engaged by the .impeller blades is driven towards the right, some of it passing into the impeller casing under the cutoff point to form a further mixture ot gas das and liquid and some of it being rejected by passing above the cutoff point 40.

When the pump has evacuated the suction pipe, so that water flows freely into the impeller, or it is otherwise supplied with a lull complement of liquid, the velocity, density, and volume of discharge issuing from the volute opening 43, and the communicating opening 4l! is such as to divert the streams illustrated at C and D in Figure 6 to the complete exclusion of reentry.

The circulation of liquid then instead of be coming reentry isl converted merely into an eddy y within the separator I6. The discharge from the impeller casing passes through the separator and out the discharge opening 20, whereas the eddy maintained 'inside the separator I6 keeps the channels 12 and 'I3 swept clear of sand, gravel and debris. Such solids are swept into the issuing stream and are carried out of the separator.

While this pump is particularly suitable for the construction industry in the form herein shown, it is to be understood that it is not to be limited to that service, and that by suitable modicaticns at once apparent to those skilled in the art, the pump may be designed for any particular service as to pressure, volume, rate of priming, etc.

The mode of operation of the closed impeller type shown in Figure 5 is substantially identical with that described in connection with Figures 2, 3 and 4. Compensation to control reentry and -l'iydraulic stoppage of reentry during liquid pumping are accomplished in the same manner as above described. i

I do not intend to 4be limited to the specic' i an impeller casing providedI with a central inlet i and a peripheral volute, a separator disposed substantially in the plane of the impeller casing and having a bottom wall, said volute having a part terminating in the separator above the bottom wall and opening directly into the separator, said separator and casing having communication 4through said opening, the sides of said separator being disposed in substantially parallel planes on each side of and spaced away from the sides of said part of the volute to define two open channels for priming Water leading from back of' the said part of the volute to the sides of said opening and below the top of said opening.

2. In a self-priming centrifugal pump the coni-5 bination of a body comprising a single casting constituting an intake trap, a separator, and an impeller casing, the trap and separator having substantially the same breadth andbeing of substantially the same breadth asthat of the impeller casing, said trap and separator having inlet and outlet openings respectively on substantially the same level, the capacity of the trap being not substantially more than that of the separator with the impeller chamber, there -being a vertical planar wall betweenthe trap and the separator, the impeller chamber having at one side an axial opening communicating with the bottom of the inlet trap and having at the other side an opening for the insertion of the impeller, said impeller casing having an integral extension projecting into the separator and 'disposed above the bottom wall ofthe separator, `said integral extension being curved longitudinally and arched transversely and having a discharge opening extending above the said bottom wall of the separator, said bottom wall having a portion lying at the side of and below said arched extension and serving to conduct liquid for reentry directly from the separator into the impeller below the top of said discharge opening, said impeller casing and said separator lying in substantially the samer vertical plane, an immember, said closure member comprising a plate apertured for the impeller shaft, said plate closing the impeller casing, said plate comprising a cylindrical extension and a clamping ilange for clamping said member to the pump body, the junction of the separator and the impeller casing forming at one side of the separator a cutoii for the impeller.

3. Ein a pump of the class described, a separator, a pump casing, an impeller in the casing, said casing and separator having a common wall with an opening therethrough through which they communicate, the periphery of the impeller projecting substantially into said opening, there being a longitudinally inclined arched wall extending over a part of said opening, said arched wall joining the common wall on three sides and leaving .on the fourth side a discharge opening Vwhich directs a mixture of gas and liquid through a part of said `first named opening and delivers it into the separator above the liquid in the bottom vof the separator, said arched wall deiining with a side wall of the separator an open-topped channel for leading priming liquid from the lower part of the separator and from the back of said arched wall to a part of said` opening at a level below the issuing stream of mixture whereby during priming the reentry of liquid and the discharge of mixture are conductedby independent streams with minimum interference.

4. The combination of claim 3 wherein said pump casing has a volute .channel and said arched wall forms an extension of the volute.

5. In a self-priming centrifugal pump an impeller casing having a central inlet and a peripheral outlet,a separator chamber enclosing said outlet and adapted to maintain a body of liquid over said outlet sealing the same, an impeller mounted for rotation in said casing, said casing having a channel for fluid to be pumped lying in the plane of vrotation of the impeller, one part ofthe channel having substantially no more than mechanical clearance with the blades of the impeller and ,at another point diverging from the periphery of the impellerV substantially in the plane' of the imepller wherebyit is adapted to conduct the mixture of gas and liquid discharged during priming radially away from the ends of the blades of the impeller to cause a region of reduced pressure substantially at the periphery of the impeller, said region of reduced pressure being exposed laterally to the lower part of the body of liquid maintained in said separator whereby liquid iiows from the lower part of said body of liquid laterally into the region of reduced pressure radially under said issuing stream of mixture and enters the -impeller to be mixed with gas in the impeller casing during priming.

6. In a self-priming centrifugal pump of the volute type, an impeller casing hayingla central inlet and a 4peripheral tangential outlet, an impeller mounted for rotation in said casing, said casing having a channelfor iiuid to be pumped i defined by a peripheral volute and terminating at its small end in a cutoff which cooperates with the outer periphery of the impeller to compel a division of the flow of uid which is moved by the action of the impeller, and terminating at its large end in a mouth adjacent to and radially outside the said cutoff, a separator chamber into the lower end of which said outlet extends and terminates, said separator chamber being adapted to maintain a charge of sealing and priming liquid over said outlet during priming, the larger end of the volute adjacent the outlet conducting the stream of mixture of gas and liquid expelled from the said casing into the separator byy the impeller during priming, in a path which diverges radially from the peripheryof the impeller where- -by there is produced at the periphery of the impeller between the divergent stream of mixture and the cutoff a region of pressure lower than the pressure of the liquid in the adjacent part of the separator, said region being laterally open to the liquid in the lower part of the separator whereby liquid of less gas content than the mixture is caused to flow into the periphery of the impeller in advance of said cutoff, said cutoff serving to divide the liquid acted upon by the impeller in said region of reduced pressure to cause a part thereof to enter the impeller casing and to be mixed with liquid and the other part to Ee rejected into the separator.

HARRY E. LA BOUR. 

